UIDL ⇝

Unique ID Listing

(UIDL) A system used by POP3electronic mailservers to uniquely identify a mail message. Normally, a
message is identified by its position in the list of messages
but this will change when an earlier message is deleted. The
UIDL is a fixed string of characters which is unique to the
message. The UIDL of a message never changes and will never
be reused, even when the message has been deleted from the
user's mailbox.